Cataract extraction is one of the most commonly performed surgical procedures in the world. A cataract is the opacification of the crystalline lens or its envelope—the lens capsule—of the eye. It varies in degree from slight to complete opacity that obstructs the passage of light. Early in the development of age-related cataract the power of the lens may be increased, causing near-sightedness (myopia), and the gradual yellowing and opacification of the lens may reduce the perception of blue colors as those wavelengths are absorbed and scattered within the crystalline lens. Cataract typically progresses slowly to cause vision loss and are potentially blinding if untreated.
Treatment is performed by removing the opaque crystalline lens and replacing it with an artificial intraocular lens (IOL). An estimated 3 million cases are presently performed annually in the United States and 15 million worldwide. This market is composed of various segments including intraocular lenses for implantation, viscoelastic polymers to facilitate surgical maneuvers, disposable instrumentation including ultrasonic phacoemulsification tips, tubing, and various knives and forceps.
Modern cataract surgery is typically performed using a technique termed phacoemulsification in which an ultrasonic tip with associated irrigation and aspiration ports is used to sculpt the relatively hard nucleus of the lens to facilitate it removal through an opening made in the anterior lens capsule termed anterior capsulotomy or more recently continuous curvilinear capsulorhexis (CCC). Finally, a synthetic foldable intraocular lens is inserted into the remaining lens capsule of the eye through a small incision.
One of the most technically challenging and critical steps in the procedure is making the capsulorhexis. This step evolved from an earlier technique termed can-opener capsulotomy in which a sharp needle was used to perforate the anterior lens capsule in a circular fashion followed by the removal of a circular fragment of lens capsule typically in the range of 5-8 mm in diameter. This facilitated the next step of nuclear sculpting by phacoemulsification. Due to a variety of complications associated with the initial can-opener technique, attempts were made by leading experts in the field to develop a better technique for removal of the anterior lens capsule preceding the emulsification step.
The concept of the continuous curvilinear capsulorhexis is to provide a smooth continuous circular opening through which not only the phacoemulsification of the nucleus can be performed safely and easily, but also for easy insertion of the intraocular lens. It provides both a clear central access for insertion, a permanent aperture for transmission of the image to the retina by the patient, and also a support of the IOL inside the remaining capsule that would limit the potential for dislocation.
Problems may develop related to inability of the surgeon to adequately visualize the capsule due to lack of red reflex, to grasp it with sufficient security, to tear a smooth circular opening of the appropriate size and in the correct location without creating radial rips and extensions. Also present are technical difficulties related to maintenance of the anterior chamber depth after initial opening, small size of the pupil, or the absence of a red reflex due to the lens opacity. Some of the problems with visualization have been minimized through the use of dyes such as methylene blue or indocyanine green. Additional complications arise in patients with weak zonules (typically older patients) and very young children that have very soft and elastic capsules, which are very difficult to controllably and reliably rupture and tear.
Many cataract patients have astigmatic visual errors. Astigmatism can occur when the corneal curvature is unequal in all directions. Nowadays, IOLs are used to correct for astigmatism but require precise rotational and central placement. Additionally, IOLs are not used for correction beyond 5D of astigmatism, even though many patients have more severe aberrations. Higher correction beyond 5D is required to reshape the cornea to become more spherical. There have been numerous approaches, including Corneaplasty, Astigmatic Keratotomy, Corneal Relaxing incision (CRI) and Limbal Relaxing Incision (LRI). Except the Corneaplasty, all procedures are done by placing corneal incisions in a well defined manner and depth to allow the cornea to change shape to become more spherical. Nowadays, these delicate cuts are placed manually with its implication on its limited precision.
But, not only cuts are desired for ophthalmic therapies. There is also the need for more gentle modifications of the eye tissue which result in weakening of the tissues mechanical properties and or changes of the optical properties of the treated tissue. In this case, the effect should be gentle enough to allow structural modifications of the eye tissue without mechanical disruption. Ding et al. (IOVS, 2008 (49), 12, pp 5532-5539) showed modification of corneal tissue with sub-rupture femtosecond laser pulses and could demonstrate changes in the refractive index by about 1% by applying diffraction patterns into the corneal tissue. The practical application of Ding's technique is although limited by the need to apply 100,000,000 laser pulses per cubic micrometer of treated tissue.
Vogel et al. (US 2010/0163540 A1) describes a method for machining and cutting of transparent material with temporal smooth laser beams to generate a low density plasma without the formation of plasma luminescence. In the teaching, they describe that linear absorption of the exposed material is especially to be avoided as it leads to the random generation of seeding electrons which in turn generates a stochastic variation in the plasma threshold. Additionally, they describe that the low density plasma formation is always associated with the formation of cavitation bubbles.
This is in strong contrast to the present invention in which two working regimens are described. It was discovered that using a laser wavelength that has some linear absorption in the target tissue enables to create extremely low threshold effect. Additionally, a temporal smooth pulse shape is not required in the current invention. Also, the formation of a cavitation bubble is not desired in one embodiment of the invention as the effect is induced by linear absorption enhanced photodecomposition. Also, Vogel's data show that there is still more than one order difference in achieving plasma formation when comparing IR femtosecond lasers and 355 sub-ns laser. In our embodiment, due to the use of the linear absorption of tissue intrinsic chromophores (or via the addition of exogenous chromophores) the energy threshold for the 355 nm sub-nanosecond laser is even slightly lower when compared to femtosecond laser pulses using the same numerical aperture optics.
Braun et al. (DE 198 55 623 C1) describes a method for precise machining inside of glass using a laser with wavelength outside the transmission plateau of the glass. This laser is then used to specifically create material defects inside the glass without comprising the surface. This method allows them to place material defects closer to the surface without damaging the surface itself. No surface effects are described. It also does not create any cavitation event as its used only on glass in which no cavitation bubble is formed.
Koenig et al. (WO 2007/057174) claims a system for the surgical intervention of the eye by using femtosecond laser pulses in the UV spectral range. In his teaching, he describes the use of higher numerical apertures of 0.8 for his invention which lowers the threshold significantly into the nanoJoule regimen. But, he makes the transfer of this system into a useable product so difficult as it is optically difficult to have these numerical apertures combined with a wide scan ranges of 6 to 10 mm typically used for ophthalmic applications. Also, the generation of femtosecond UV laser pulses is technically challenging.
Therefore, methods, techniques, and an apparatus to advance the standard of care of the ophthalmic patient are needed.